The three important layers in the Internet of Things (IoT) are the perception layer, the internet layer and the application layer, and the most important component in the perception layer is the sensor. Therefore, as the technology IoT continues to develop, the demands for sensors increase correspondingly. Currently, sensors that are miniature, low in power consumption and highly sensitive are the most demanding in applications, especially for wearable or mobile phone devices.
Presently, the most fundamentally and customarily used sensors are gas, temperature, and humidity sensors, wherein in most gas sensors, a temperature sensor and a humidity sensor are integrated on an extra system board for performing calibrations under the different ambient conditions to provide a better accuracy. Alternatively speaking, most gas sensors are arranged with temperature and humidity sensors. However, for wearable or mobile phone devices, the space for accommodating sensors is very limited; hence, to miniaturize and integrate sensors of various functions in a same fabrication process has been actively pursued by the relevant industries.